In wireless communication systems, transmission techniques involving multiple antennas are often categorized as open-loop or closed-loop depending on the level or degree of channel response information used by the transmission algorithm. Open-loop techniques do not rely on the information of the spatial channel response between the transmitting device and the receiving device. They typically involve either no feedback or the feedback of long term statistical information that base unit may use to choose between different open loop techniques. Open-loop techniques include transmit diversity, delay diversity, and space-time coding techniques such as the Alamouti space-time block code.
Closed-loop transmission techniques utilize knowledge of the channel response to weight the information transmitted from multiple antennas. To enable a closed-loop transmit array to operate adaptively, the array must apply the transmit weights derived from the channel response, its statistics or characteristics, or a combination thereof. There are several methodologies for enabling closed-loop transmission. These are discussed in the context of the downlink of a cellular communication system in which a base station (BS) with multiple antennas transmits to a mobile station (MS) having one or more receive antennas and one or more transmit antennas. The MS may not necessarily have a number of transmit antennas as receive antennas. Exemplary closed-loop methodologies include adaptive transmit beam-forming, closed-loop single-user MIMO, and closed-loop multi-user MIMO. In these examples, the transmitter applies weighting coefficients that are derived according to an optimization algorithm to control characteristics of the transmitted signal energy.
One methodology for enabling closed-loop transmission is uplink channel sounding (ULCS) wherein the MS transmits a known pilot sequence on the uplink. In a time division duplexing (TDD) system in which the BS array is calibrated for transceiver reciprocity, the ULCS signal enables the BS to estimate the uplink channel response and convert that estimate to a downlink channel estimate, which is used to compute the transmit weights under a multipath channel reciprocity assumption. In a frequency division duplexing (FDD) system, the ULCS signal can be used by the BS to compute the direction of arrival (DOA) of the signal from the MS, wherein the DOA is used to compute a set of transmit weights when the base station array is calibrated for both uplink and downlink frequencies.
Another methodology for enabling closed-loop transmission is direct channel feedback (DCFB), wherein the MS measures the downlink channel response and encodes that channel response as an analog signal to be conveyed on the uplink. The downlink channel response estimates are encoded along with known pilot signals that enable the BS to estimate the analog values of the downlink channel estimates. DCFB can be applied to both FDD and TDD systems.
Another methodology for enabling closed-loop transmission is codebook index feedback in which both the BS and MS maintain a finite codebook of possible transmit weight vectors or matrices, depending on the number of simultaneous transmit beams being formed. The MS measures the downlink multi-antenna channel response and computes the transmit weight vector or matrix that is best used to transmit information. The MS then transmits the index into the codebook back to the BS, where the index into the codebook is often called a Precoding Matrix Index (PMI). The BS uses the transmit weight vector or matrix corresponding to the index fed back by the MS. Codebook index feedback can be applied to both FDD and TDD systems.
Another methodology for enabling closed-loop transmission is quantized channel feedback wherein the MS measures the downlink channel and quantizes the channel response into digital form in which some number of bits are used to convey gain information and some number of bits are used to convey phase information for a given channel coefficient. Variations on this methodology are also possible.
In principle, the various methodologies for enabling closed-loop transmission provide good performance. However, there are implementation problems and constraints that often hinder performance. For example, the effectiveness of ULCS and DCFB can be limited by a low Signal to Noise Ratio (SNR) on the uplink due to the MS transmit power being much less, for example, 9-18 dB less, than that of a BS. With a low SNR uplink, ULCS and DCFB are effective at providing channel response information for a narrow band of the broadband channel due the ability of the MS to concentrate transmit power on the narrow band. However to provide channel response information for the entire band, the low transmit power of the MS (relative to the BS) often limits the ability of ULCS or DCFB techniques to provide good closed-loop performance on the DL. Also, quantized feedback may require large amounts of feedback but with worse performance than PMI feedback. Furthermore, PMI feedback can in many cases be inferior to ULCS due to a form of quantization error resulting from the finite number of transmit weight vectors/matrices to choose from. Thus there is a need for an improved feedback methodology to enable closed-loop transmit antenna array techniques in wireless communication systems.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.